The present invention relates to cutaneous gas sensors and is directed more particularly to a method and apparatus for removing the metal deposits which become plated on the electrodes of such sensors during normal operation.
Among the non-invasive patient monitoring devices which have been developed recently is an instrument known as a cutaneous gas sensor. Such gas sensors make use of known gas detection techniques to measure the partial pressure of a gas, such as oxygen or carbon dioxide, which diffuses outwardly through pores in the patient's skin. Cutaneous gas sensors have also been developed which simultaneously measure the partial pressures of both oxygen and carbon dioxide. One cutaneous gas sensor of the latter type is described in "Cutaneous Blood Flow and its Relationship to Transcutaneous O.sub.2 /CO.sub.2 Measurements", by A. V. Beran, et al., "Critical Care Medicine", Vol. 9, No. 10, pages 736-741 (1981).
During operation, cutaneous gas sensors that measure oxygen tend to produce oxygen partial pressure readings which slowly increase with time. The principal reason for this increase has been found to be the plating of anode material, usually silver, onto the cathode electrode of the sensor. This plating increases the apparent oxygen partial pressure by increasing the effective area of the cathode electrode. In order to eliminate the errors that are associated with this increase, it has been the practice to visually inspect the electrodes of cutaneous gas sensors, and to clean the same when the need for deplating became apparent.
Prior to the present invention the plated anode material was commonly removed by rubbing the cathode electrode with emery paper. While this method of cleaning the cathode electrode removed the plated anode material, it often damaged the cathode electrode and surrounding structures. The use of emery paper can, for example, change the curvature of the surface of the electrode or produce flat spots thereon. Such changes in curvature or flat spots are undesirable because they tend to reduce the sensitivity of the gas sensor.
Another method that was used experimentally in an attempt to remove plated anode material prior to the present invention involved establishing a repeating sequence of deplating voltages between the anode and cathode electrodes. One such sequence included the application of -1.2 volts between the electrodes for four second periods at 20 second intervals, followed by the application of +1.2 volts for four second periods at 20 second intervals. The use of this method, however, resulted in unstable oxygen partial pressure readings and in the incomplete removal of plated anode material.